Trench-type MOSFET's have been developed. A trench-type MOSFET is a vertical MOSFET which includes trenches and gate electrodes buried in the respective trenches such that channels are formed in the side wall portions of the trenches. The trench-type MOSFET is advantageous to reduce the cell pitch and the on-resistance per a unit area. As far as lateral MOSFET's mountable on power IC's are concerned, some trench-type structures thereof have been proposed. However, the trench-type lateral MOSFET's have not been put into practical use yet.
The trench-type lateral MOSFET's that have been previously proposed include a top-drain-trench-type RESURF DMOS transistor having trenches formed between the sources and the drains thereof and gates formed in the respective trenches (Japanese Unexamined Laid Open Patent Application H06-97450). Another reported trench gate structure buries polycrystalline silicon gates in the trenches formed in the same manner as described above (ISPSD 2000, pp. 47-50). A lateral power transistor proposed includes drains, sources and trenches formed in a substrate, and gates formed on the surfaces of the portions of the substrate between the sources and the trenches, in which channels are formed (Japanese Unexamined Laid Open Patent Application H07-74352).
A transistor has also been proposed that includes trenches formed from the surface of the drain drift region and filled with an insulator or a semi-insulator, and gate electrodes extended onto the trenches (cf. Japanese Unexamined Laid Open Patent Application H08-97441). Furthermore, a transistor, the edge portions of the gate electrodes thereof are above the trenches, has been proposed (cf. ISPSD 1999, pp. 73-76).
The transistor disclosed in the Japanese Unexamined Laid Open Patent Application H06-97450 is manufactured in the following way. The oxide films formed on the inner walls of the trenches are selectively oxidized to form non-uniform oxide layers. Then, gate oxide films are formed by wet-etching the thin oxide layers. Finally, trenches are filled with gate polycrystalline silicon. As described above, the transistor disclosed in the Japanese Unexamined Laid Open Patent Application H06-97450 is manufactured through a complicated manufacturing process including many steps.
The transistor having a trench gate structure and reported in the ISPSD 2000 exhibits a low breakdown voltage of lower than 20 V. This transistor has been proposed to reduce the channel resistance, which is the predominant component of the on-resistance thereof. Since the source electrode and the drain electrode of the transistor reported in the ISPSD 2000 do not exhibit any field plate effect as FIG. 1 described in the report of the ISPSD 2000 indicates, the reported trench gate structure is not suitable for obtaining a transistor exhibiting a breakdown voltage of the several hundreds volt class.
Since the source electrode and the drain electrode of the transistor disclosed in the Japanese Unexamined Laid Open Patent Application H07-74352 do not exhibit any field plate effect as FIG. 2 of the above identified publication indicates, this transistor is not suitable for obtaining a breakdown voltage of 200 V or higher.
In the transistor disclosed in Japanese Unexamined Laid Open Patent Application H08-97441, the extension of the gate electrodes above the trenches thereof may exhibits field plate effects. However, the source electrode and the drain electrode of this transistor do not exhibit any field plate effect as FIG. 1 of the above identified publication indicates. Therefore, the transistor disclosed in Japanese Unexamined Laid Open Patent Application H08-97441 is not suitable for obtaining a breakdown voltage of 200 V or higher.
Since the source electrode and the drain electrode of the transistor disclosed in the report of the ISPSD 1999 do not exhibit any field plate effect, this transistor is not suitable for obtaining a breakdown voltage of 200 V or higher.
Since the surface impurity concentration in the device exhibiting a breakdown voltage of the 700 V class is low, the ions contained in the mold resin may adversely affect the device characteristics. Specifically, the device characteristics may be deteriorated by the reliability tests conducted in a high-temperature and high humidity environment due to the ions contained in the mold resin.
In view of the foregoing, it would be desirable to provide a semiconductor structure which facilitates providing a lateral trench-type MOSFET exhibiting a high breakdown voltage of 200 V or higher and preventing the manufacturing steps thereof from greatly increasing. It would further be desirable to provide a semiconductor structure which facilitates providing a lateral trench-type MOSFET exhibiting a breakdown voltage of the 700 V class and preventing the ions in the mold resin from adversely affecting the device characteristics thereof.